The drawn and ironed cans are usually produced by punching a blank from a metal sheet, forming a cup by drawing the blank, mounting the drawn cup on a punch by using a drawing device which includes the punch and ironing dies of a plurality of stages, and inserting the cup together with the punch into the ironing dies so as to be finished having a predetermined can wall thickness and a can height. In the drawing and ironing, there are used large amounts of a lubricant and cooling water for lubrication and cooling during the working.
From the standpoint of preserving environment and further decreasing the weight of the can body in recent years, attempts have been made to further decrease the thickness of the can wall by ironing in the can body made of a resin-coated metal sheet that has hitherto been chiefly worked by drawing. As the thickness of the can wall is further decreased, however, the can wall tends to be easily broken making it difficult to decrease the thickness of the can wall, i.e., making it difficult to decrease the cost to a sufficient degree by reducing the weight.
In order to solve the above problem, Japanese Patent No. 3582319 teaches a method of producing seamless cans by inserting a blank holder in a drawn and formed body, advancing a punch into a cavity of a die while holding the bottom of the draw-formed body onto a flat surface of the die by using the blank holder, decreasing the thickness of the can wall relying upon the bend-elongation at the working corner while bringing the outer surface of the can wall of the draw-formed body into intimate contact with the working corner of the die of a small radius of curvature, and further decreasing the thickness of the can wall by the ironing based upon both the punch and the front end of the working corner in the ironing or the ironing portion ahead of the front end thereby to form a first seamless can longer than the drawn and formed body, followed by the working by using a punch provided with a tapered portion connecting to an upper end of a cylindrical portion of the punch, the tapered portion inclining rearward and inward at 0 degree, one minute to 0 degree, 30 minutes so as to be corresponded to a thickness profile of the drawn and formed body in the direction of height in which the thickness gradually increases toward the upper side of the can wall, so that the amount of reduction of thickness becomes equal in the direction of height after the ironing of the can wall of the drawn and formed body.
In the above method of producing seamless cans, the drawing and ironing is effected by using a re-drawing die and an ironing die in combination. Particularly, the opening of the formed can has not been subjected to the ironing but is simply subjected to half way of the drawing. Therefore, the thickness cannot be decreased highly precisely up to the opening.
As the ironing method, further, JP-A-2003-19518 discloses an ironing method for decreasing the thickness of the cup wall by bringing a metal cup supported by a punch into mesh with a die for ironing, wherein the ironing portions of the preceding stage and the succeeding stage are arranged in succession maintaining a distance of 3 to 20 mm between the lands, and the ironing portion of the preceding stage effects the ironing of an amount of not smaller than 20% of the whole amount of ironing effected by the ironing portion of the preceding stage and by the ironing portion of the succeeding stage.
However, an attempt to form a drawn and ironed can having a further decreased thickness by the above ironing method is accompanied by a problem of breakage in the can wall.
The present inventors have conducted a keen study to investigate the cause of breakage of the can wall and have discovered the following facts. That is, when the ironing is effected by using the ironing dies of the preceding stage and the succeeding stage arranged in succession as in the above ironing method, a shock line (locally thinned portion) occurs at a position corresponding to an end on the side of the can wall at a stepped portion formed between the flange-forming portion and the can wall due to the arrival of the die of the preceding stage, and the can wall is broken. Besides, the thickness of the can wall near the stepped portion becomes greater than a clearance established between the punch and die, making it difficult to obtain a thickness profile as designed.